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Journal of the Geological Society

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Discussion on the geological evolution of the trachytic caldera volcano Menengai, Kenya Rift Valley

DR W. B. JONES writes: I would like to add some comments to Leat’s description of Menengai as a result of my own fieldwork to the northwest of that volcano.

Lower Menengai Tuff

Leat states that two ash flows outcrop in the caldera walls, and his map (fig. 2) shows an outcrop of the first ash flow about 20 km northwest of the caldera. At this locality about 20m of largely reworked tuffs are capped by an ash flow 7 m thick. The base of the outcrop contains a rich Acheulian artefact horizon showing reverse magnetization (Bishop 1978) and archaeological evidence for an age of about 0.7 Ma (J. Gowlett, pers. comm.). It is therefore very unlikely that this ash flow is the equivalent of the lower ash flow in the caldera wall since the latter is dated by Leat at about 29,000 years. The tuffs at the artefact site, the Lower Menengai Tuff of Jones (1975) and Jones & Lippard (1979), also occur northwards to beyond the equator and within the caldera of Kilombe, and seem to have been derived from the southeast. K-Ar dates of 0.33 ± 0.01 Ma (Jones & Lippard 1979) and 0.18 ± 0.01 Ma (Leat 1984) for Menengai shield trachytes indicate that the present Menengai volcano is too young to be the source of the Lower Menengai Tuff but a possible source may lie to the northwest of the present caldera. An E–W ridge of trachyte downfaulted on its southern side, and an

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